The present invention relates to a modulator for generating a waveform by digital processing in a communication apparatus and, more particularly, to a modulation system for making the amplitude of a transmission waveform uniform.
In an apparatus for generating an analog waveform signal to be transmitted by a modulator using digital processing in data transmission, a modulator for outputting an analog signal waveform with reference to data of a memory device using an input signal as an address is known (see, e.g., Japanese Patent Laid-Open No. 62-85538). FIG. 1 shows a modulator disclosed in this reference. This modulator comprises two memory devices 11 and 12 for generating quadrature and in-phase components of an analog waveform like the one shown in FIG. 2.
The modulator shown in FIG. 1 comprises the two memory devices 11 and 12 for generating quadrature and in-phase components of an analog waveform, and two D/A converters 13 and 14 for converting digital values from the memory devices 11 and 12 into analog values. The memory device is generally large in circuit scale. As the precision of an output signal increases, the circuit scale further increases, posing a problem in downsizing the modulator. The D/A converter greatly consumes the power for a higher speed and higher precision, posing a problem in reducing power consumption of the modulator.